Dr. Jonathan L. Larson provides a monthly feature on pests that may be a problem in your home or on your acreage. This month Jonathan tells us how to deal with Lilac-Ash Borer.
April 2017. Not all insects that attack your ash tree are the dreaded emerald ash borer. We actually have quite the suite of native ash tree pests. One example from this group would be the lilac-ash borer which can attack several ornamental plants but can also be easily controlled through proper integrated pest management tactics.
The lilac-ash borer is part of a group of moths known as the clearwing moths. Clearwing moths differ from the other moths in that their wings aren’t coated with powdery scales, so there is little to no color or patterns on them. This lack of wing coloration, their day flying activity, and their body shape and coloration all combine to make these moths appear to be a wasp or hornet, offering them some degree of protection from predation. The lilac-ash borer in particular looks a lot like a paper wasp (Fig. 1). The adult moth has a wingspan of about an inch and a quarter and is actively flying from April until July here in Nebraska.
The moth stage is not the damaging stage of this pest though. As a caterpillar the lilac-ash borer feeds under the bark of ash trees, lilac bushes, and privet. Female moths lay their eggs in the bark crevices of stressed plants and the caterpillar burrows just under that initial layer to begin feeding. Eventually it will move further into the sapwood. During the summer they will eat and grow and will push their fecal material out through a hole in the plant. You can often find piles of sawdust like caterpillar poo at the base of an infested tree. They spend the winter as a caterpillar and pupate in the spring. Right before pupation they cut a small hole in the tree that they will use to emerge as an adult. In these holes you can often find their left behind pupal skin (Fig. 2), another great symptom to keep an eye open for. There is only one generation per year.
Proper management of this pest depends on dedicated monitoring. There are pheromone lures available that will help you or the client to better predict when to spray in order to thwart this pest. Pheromone traps should be placed out in late April or when lilac is in full bloom, whichever comes first. When you catch at least five borers you can start to take chemical action. You can apply permethrin or bifenthrin to the trunk and largest limbs to protect the plant. Systemic neonicotinoid treatments are not effective against this particular pest. You can reduce the attractiveness of your plant to borers by ensuring proper pruning and watering and avoiding wounds to the plant (such as mower damage).